PROJECT SUMMARY Currently there is no treatment to stop or slow down the progression of osteoarthritis (OA) available. Therefore, the discovery of novel mechanisms or factors that protect articular chondrocytes against catabolic events and ultimately slow down cartilage degradation during OA progression is highly needed. Recently, cytokine receptor-like factor 1 (CLRF1) and cardiotrophin-like cytokine (CLC), two cytokines belonging to the IL-6 or gp130 cytokine superfamily, have become the matter of intense investigations because of their potential activities in adult biology, degenerative and regenerative conditions in a wide range of organ systems. CRLF1 can signal alone as a homodimer or as a heterodimeric complex together with CLC. More importantly, CRLF1 homodimer signaling has been show to protect neurons against oxidative stress, whereas CRLF1/CLC heterodimer signaling has been shown play major roles in adult pathologies of various tissues. Previous studies have shown that CRLF1 is being expressed by bone and cartilage cells. Very little, however, is known about the role of these two cytokines in cartilage homeostasis and OA pathology. Our exciting preliminary findings showing increased expression of CRLF1 during early stages of OA, the induction of CLC expression in articular chondrocytes under inflammatory conditions, the stimulation of catabolic events in human articular chondrocytes by the CRLF1/CLC complex, and the chondro-protective effects of CRLF1 alone supports our hypothesis that CRLF1 and CLC play important roles in cartilage homeostasis and OA pathology. Specifically, we propose that while the heterodimeric CRLF1/CLC complex stimulates catabolic events in articular chondrocytes, exogenous CRLF1 despite the presence of CLC protects chondrocytes in an inflammatory environment. To test our hypothesis we are proposing two aims. In Aim 1, we will establish the optimal conditions under which exogenous CRLF1 protects articular chondrocytes and cartilage explants in vitro when cultured under inflammatory conditions, and specifically determine whether exogenous CRLF1 as a homodimer, via promoting the internalization and degradation of the CRLF1/CLC/CNTFR complex by binding to SORLA, or both protects chondrocytes. In addition, we will determine whether the CRLF1/CLC heterodimeric complex is sufficient to induce catabolic events in chondrocytes and/or accelerates catabolic events in IL-1b-treaded chondrocytes. Using the findings obtained in Aim 1, we will determine in Aim 2 whether CRLF1 can be used in vivo to protect articular cartilage against degradation after intra-articular IL-1? injection or in a surgical-induced OA model in mice, and determine whether CRLF1/CLC complex contributes to cartilage degradation in vivo. The successful completion of this proposal will establish the role of two cytokines CRLF1 and CLC in articular cartilage homeostasis and OA pathology. We expect that the understanding of how these two cytokines individually or as a complex affect articular chondrocytes will provide novel therapeutic targets for the treatment of OA.